There is considerable evidence supporting a causal role for T- lymphocytes, monocytes, and monocyte-derived macrophages in the initiation, progression, and complications of the atherosclerosis in man as well as in experimental models. Plaque rupture is thought to be the trigger event for acute coronary syndromes in man, and several mouse models of plaque rupture have recently been described. Lesion macrophages in particular have been implicated in plaque rupture by releasing de-stabilizing proteases. Notably, there is evidence for continued recruitment of circulating monocytes into arterial lesions in experimental atherosclerosis, particularly in the rupture-prone shoulder. Moreover, adhesion receptors may also regulate critical functions of emigrated leukocytes resident in lesions, including activation and survival. Thus, the adhesion molecules that mediate monocyte trafficking into the arterial wall a potentially attractive target in advanced as well as early disease. Studies to date of human and experimental lesions suggest a significant role for endothelial VCAM-1 and its major leukocyte counter-receptor VLA-4 a4B1) in the early phase of disease, but have not examined the role of VLA-4 or VCAM-1 in the progression of established atherosclerosis or its late complications such as plaque rupture. Since antagonists of VLA-4 have already progressed to clinical trials in other indications, the importance of VLA-4 and VCAM-1 interactions in the progression and late complications of atherosclerosis is a clinically relevant question as patients are most often identified in this stage of the disease. We hypothesize that the VLA-4 and VCAM-1 play important roles in monocyte and T-lymphocyte recruitment to advanced as well as early lesions and that disruption of these adhesion pathways will reduce progression test this of established lesions and prevent plaque rupture. In order to test this hypothesis, we will utilize a recently developed mouse model in which interferon-induced Cre-loxP-mediated deletion of the (4 gene can be achieved at any time post-natal. These (4-deleted animals will be studied in both the ApoE-/- (Aim 1) and LDRL-/- (Aim 2) background, allowing us to define for the first time the contribution of VLA-4 in the progression of established lesions and plaque rupture as well as in lesion initiation. To complement the VLA- 4 studies, we will also examine the role of VCAM-1 in lesion initiation/progression in a model of conditional knockout of endothelial VCAM-1 (Aim 3).